1. Field of the Invention
This invention broadly relates to a process for treating methanol-containing waste gas streams, such as encountered in a paper (pulp) mill. More particularly, this invention provides a method wherein a gas stream, such as from a paper pulp mill containing methanol, and other waste products, including methyl mercaptans, is passed in contact with a catalyst comprising a supported or unsupported bulk metal oxide in the presence of an oxidizing agent. In a preferred embodiment, the gas stream is contacted with the catalyst, in the presence of the oxidizing agent, for a time sufficient to convert at least a portion of the methanol to formaldehyde (CH.sub.2 O).
2. Description of Related Art
Pulp mills that chemically digest wood sources, such as by using the Kraft process, generate a significant amount of by-product methanol, in addition to other potential pollutants such as reduced sulfur compounds, higher alcohols, terpenes, acetone, amines, acetaldehyde and methyl ethyl ketone. Significant methanol emissions are encountered during the overall pulping and paper-making operation, particularly from the following pulp mill sub-systems: pulp digesters, blow heat recovery units, and multiple-effect evaporators. In the past, this methanol was often discharged into the air or directly into wastewater. With growing environmental concerns over pollution caused by these past practices, however, more stringent emission requirements have evolved. As a consequence, processes must be developed to dispose of the methanol, and the other pulp mill by-products, in a more environmentally satisfactory manner. At the present time, two alternatives have been suggested as being suitable for complying with regulatory standards. Garner, Jerry, Pulp & Paper, (August 1996):59-62. In both approaches, the various waste steams generated in a pulp mill containing by-product methanol, and a variety of the other noted compounds, are first consolidated into a single condensate stream.
In a first alternative, this consolidated condensate stream is fed to a steam stripping column which is operated in a way to remove and concentrate, in the gas phase, a major portion of the pollutants from the liquid condensate. Over 80-90% of the methanol of the consolidated feed steam is removed in the stripping column, while the methanol concentration is increased from about 0.1-0.5% in the liquid feed to about 35-55% in the steam stripper gaseous overhead.
This methanol-containing stripper overhead is then treated by indiscriminate (noncatalytic) oxidation (incineration) to produce a waste gas that can be safely discharged into the environment. To safely complete the oxidation of the gas constituents, the incineration process must be operated at temperatures approaching 1000.degree. C. Such operation generally requires the use of an auxiliary fuel source. In some facilities, the fuel value of the stripper overhead is recovered by using it, for example, to power a boiler or a lime kiln. In some cases, the methanol content of the stripper overhead is further concentrated by distillation to increase its value before use as a fuel.
The second alternative delivers the condensate stream to a wastewater treatment system where aerobic microorganisms use the methanol as a carbon source, converting it to carbon dioxide and water.
While highly dependent on the wood source and the basic operating conditions of a pulp mill, the consolidated waste methanol stream or condensate from a pulp mill can be expected to contain, among other materials, methanol and other higher alcohols, methyl ethyl ketone, acetaldehyde, acetone, terpenes, amines, ammonia and reduced sulfur compounds. The concentration of these materials in the aqueous condensate is enriched by steam stripping and results in a gas stream which typically contains about 40-55 wt. % methanol, 2-8 wt. % higher alcohols, such as ethanol and isopropanol, 2-8 wt. % ketones, including methyl ethyl ketone, methyl isobutyl ketone and acetone, 1-3 wt. % reduced sulfur compounds including hydrogen sulfide and methyl mercaptans, such as methanethiol (CH.sub.3 SH), dimethyl sulfide (CH.sub.3 SCH.sub.3) and dimethyl disulfide (CH.sub.3 SSCH.sub.3), about 1-2 wt. % amines, including ammonia, 1-6 wt. % terpenes, such as .alpha.-pinene and .alpha.-terpineol, and the balance (generally 40-50 wt. %) water.
Simply incinerating or biologically degrading this methanol-containing waste stream constitutes a costly and inefficient use of the inherent resources present in the stream. While procedures used to recover the fuel value of the stream seek to reduce such inefficiencies, they create their own source of problems. In particular, the variable nature of the methanol-containing stream creates the risk of operational upsets in the operation of the lime kiln or a boiler. Further, if the lime kiln or boiler operation is interrupted, then the operation of the stripper must be discontinued since there is no way to dispose the concentrated stripper overhead. As a result of these potential problems, a pulp mill likely would need to operate with a dedicated incinerator using a power boiler as back-up.
In its preferred aspects, the present invention is directed to an improved process for using by-product methanol from methanol-containing waste streams, such as encountered in a paper (pulp) mill, to produce a valuable chemical commodity, formaldehyde.